We examine the evolution of the inner dark matter (DM) and baryonic density profile of a new sample of simulated field galaxies using fully cosmological, Λ cold dark matter (ΛCDM), high‐resolution SPH+N‐Body simulations. These simulations include explicit H2 and metal cooling, star formation (SF) and supernovae‐driven gas outflows. Starting at high redshift, rapid, repeated gas outflows following bursty SF transfer energy to the DM component and significantly flatten the originally ‘cuspy’ central DM mass profile of galaxies with present‐day stellar masses in the 104.5–109.8 M⊙ range. At z= 0, the central slope of the DM density profile of our galaxies (measured between 0.3 and 0.7 kpc from their centre) is well fitted by ρDM ∝ rα with α≃−0.5 + 0.35 log10(M★/108 M⊙), where M★ is the stellar mass of the galaxy and 4 < log M★ < 9.4. These values imply DM profiles flatter than those obtained in DM‐only simulations and in close agreement with those inferred in galaxies from the THINGS and LITTLE THINGS surveys. Only in very small haloes, where by z= 0 SF has converted less than ∼0.03 per cent of the original baryon abundance into stars, outflows do not flatten the original cuspy DM profile out to radii resolved by our simulations. The mass (DM and baryonic) measured within the inner 500 pc of each simulated galaxy remains nearly constant over 4 orders of magnitudes in stellar mass for M★ < 109 M⊙. This finding is consistent with estimates for faint Local Group dwarfs and field galaxies. These results address one of the outstanding problems faced by the CDM model, namely the strong discrepancy between the original predictions of cuspy DM profiles and the shallower central DM distribution observed in galaxies.

A study of metal enrichment of the intergalactic medium (IGM) using a series of smooth particle hydrodynamic (SPH) simulations is presented, employing models for metal cooling and the turbulent diffusion of metals and thermal energy. An adiabatic feedback mechanism was adopted where gas cooling was prevented on the time-scale of supernova bubble expansion to generate galactic winds without explicit wind particles. The simulations produced a cosmic star formation history (SFH) that is broadly consistent with observations until z ∼ 0.5, and a steady evolution of the universal neutral hydrogen fraction ( H I ) that compares reasonably well with observations. The evolution of the mass and metallicities in stars and various gas phases was investigated. At z = 0, about 40 per cent of the baryons are in the warm-hot intergalactic medium (WHIM), but most metals (80-90 per cent) are locked in stars. At higher redshifts the proportion of metals in the IGM is higher due to more efficient loss from galaxies. The results also indicate that IGM metals primarily reside in the WHIM throughout cosmic history, which differs from simulations with hydrodynamically decoupled explicit winds. The metallicity of the WHIM lies between 0.01 and 0.1 solar with a slight decrease at lower redshifts. The metallicity evolution of the gas inside galaxies is broadly consistent with observations, but the diffuse IGM is under enriched at z ∼ 2.5.Galactic winds most efficiently enrich the IGM for haloes in the intermediate mass range 10 10 -10 11 M . At the low-mass end gas is prevented from accreting on to haloes and has very low metallicities. At the high-mass end, the fraction of halo baryons escaped as winds declines along with the decline of stellar mass fraction of the galaxies. This is likely because of the decrease in star formation activity and decrease in wind escape efficiency. Metals enhance cooling which allows WHIM gas to cool on to galaxies and increases star formation. Metal diffusion allows winds to mix prior to escape, decreasing the IGM metal content in favour of gas within galactic haloes and star-forming gas. Diffusion significantly increases the amount of gas with low metallicities and changes the density-metallicity relation.The intergalactic medium (IGM) contains most of the baryons in the Universe, and provides the fuel for galaxies to form stars in which metals are produced. In turn, supernovae (SNe) and galactic winds enrich the IGM with metals, while stars and active galactic nuclei (AGN) emit UV photons. This interplay between the IGM and galaxies, mediated by metal cooling in the presence of UV, regulates the formation of stars in the Universe. The evolution and enrichment history of the IGM provides a record of this interplay.

Using high resolution cosmological hydrodynamical simulations of Milky Way-massed disk galaxies, we demonstrate that supernovae feedback and tidal stripping lower the central masses of bright (−15 < M V < −8) satellite galaxies. These simulations resolve high density regions, comparable to giant molecular clouds, where stars form. This resolution allows us to adopt a prescription for H 2 formation and destruction that ties star formation to the presence of shielded, molecular gas. Before infall, supernova feedback from the clumpy, bursty star formation captured by this physically motivated model leads to reduced dark matter (DM) densities and shallower inner density profiles in the massive satellite progenitors (M vir ≥ 10 9 M ⊙ , M * ≥ 10 7 M ⊙ ) compared to DM-only simulations. The progenitors of the lower mass satellites are unable to maintain bursty star formation histories, due to both heating at reionization and gas loss from initial star forming events, preserving the steep inner density profile predicted by DM-only simulations. After infall, gas stripping from satellites reduces the total central masses of SPH satellites relative to DM-only satellites. Additionally, enhanced tidal stripping after infall due to the baryonic disk acts to further reduce the central DM densities of the luminous satellites. Satellites that enter with cored DM halos are particularly vulnerable to the tidal effects of the disk, exacerbating the discrepancy in the central masses predicted by baryon+DM and DM-only simulations. We show that DM-only simulations, which neglect the highly non-adiabatic evolution of baryons described in this work, produce denser satellites with larger central velocities. We provide a simple correction to the central DM mass predicted for satellites by DM-only simulations. We conclude that DM-only simulations should be used with great caution when interpreting kinematic observations of the Milky Way's dwarf satellites.

We present new results on the kinematics, thermal and ionization state, and spatial distribution of metal-enriched gas in the circumgalactic medium (CGM) of massive galaxies at redshift ∼ 3, using the "Eris" suite of cosmological hydrodynamic "zoom-in" simulations. The reference run adopts a blastwave scheme for supernova feedback that produces large-scale galactic outflows, a star formation recipe based on a high gas density threshold, metal-dependent radiative cooling, and a model for the diffusion of metals and thermal energy. The effect of the local UV radiation field is added in postprocessing. The CGM (defined as all gas at R > 0.2R vir = 10 kpc, where R vir is the virial radius) contains multiple phases having a wide range of physical conditions, with more than half of its heavy elements locked in a warm-hot component at T > 10 5 K. Synthetic spectra, generated by drawing sightlines through the CGM, produce interstellar absorption line strengths of Lyα, C II, C IV, Si II, and Si IV as a function of galactocentric impact parameter (scaled to the virial radius) that are in broad agreement with those observed at high-redshift by Steidel et al. (2010). The covering factor of absorbing material declines less rapidly with impact parameter for Lyα and C IV compared to C II, Si IV, and Si II, with Lyα remaining strong (W Lyα > 300 mÅ) to ∼ > 5R vir = 250 kpc. Only about one third of all the gas within R vir is outflowing. The fraction of sightlines within one virial radius that intercept optically thick, N HI > 10 17.2 cm −2 material is 27%, in agreement with recent observations by Rudie et al. (2012). Such optically thick absorption is shown to trace inflowing "cold" streams that penetrate deep inside the virial radius. The streams, enriched to metallicities above 0.01 solar by previous episodes of star formation in the main host and in nearby dwarfs, give origin to strong (N CII > 10 13 cm −2 ) C II absorption with a covering factor of 22% within R vir and 10% within 2R vir . Galactic outflows do not cause any substantial suppression of the cold accretion mode. The central galaxy is surrounded by a large O VI halo, with a typical column density N OVI ∼ > 10 14 cm −2 and a near unity covering factor maintained all the way out to 150 kpc. This matches the trends recently observed in star-forming galaxies at low redshift by Tumlinson et al. (2011). Our zoom-in simulations of this single system appear then to reproduce quantitatively the complex baryonic processes that determine the exchange of matter, energy, and metals between galaxies and their surroundings.

We present the McMaster Unbiased Galaxy Simulations (MUGS), the first nine galaxies of an unbiased selection ranging in total mass from 5 × 10 11 M to 2 × 10 12 M simulated using N-body smoothed particle hydrodynamics at high resolution. The simulations include a treatment of low-temperature metal cooling, UV background radiation, star formation and physically motivated stellar feedback. Mock images of the simulations show that the simulations lie within the observed range of relations such as that between colour and magnitude and that between brightness and circular velocity (Tully-Fisher). The greatest discrepancy between the simulated galaxies and observed galaxies is the high concentration of material at the centre of the galaxies as represented by the centrally peaked rotation curves and the high bulge-tototal ratios of the simulations determined both kinematically and photometrically. This central concentration represents the excess of low angular momentum material that long has plagued morphological studies of simulated galaxies and suggests that higher resolutions and a more accurate description of feedback will be required to simulate more realistic galaxies. Even with the excess central mass concentrations, the simulations suggest the important role merger history and halo spin play in the formation of discs.

We present results from a fully cosmological, very high-resolution, ΛCDM "zoom-in" simulation of a group of seven field dwarf galaxies with present-day virial masses in the range M vir = 4.4 × 10 8 − 3.6 × 10 10 M ⊙ . The simulation includes a blastwave scheme for supernova feedback, a star formation recipe based on a high gas density threshold, metal-dependent radiative cooling, a scheme for the turbulent diffusion of metals and thermal energy, and a uniform UV background that modifies the ionization and excitation state of the gas. The properties of the simulated dwarfs are strongly modulated by the depth of the gravitational potential well. All three halos with M vir < 10 9 M ⊙ are devoid of stars, as they never reach the density threshold for star formation of 100 atoms cm −3 . The other four, M vir > 10 9 M ⊙ dwarfs have blue colors, low star formation efficiencies, −4.5 ≤ log M * /M vir ≤ −2.5, high cold gas to stellar mass ratios, 0.2 ≤ M HI /M * ≤ 20, and low stellar metallicities, −2 ≤ [Fe/H] ≤ −1. Their bursty star formation histories are characterized by peak specific star formation rates in excess of 50 − 100 Gyr −1 , far outside the realm of normal, more massive galaxies, and in agreement with observations of extreme emission-line starbursting dwarfs by the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey. The median stellar age of the simulated galaxies decreases with decreasing halo mass, with the two M vir ≃ 2 − 3 × 10 9 M ⊙ dwarfs being predominantly young, and the two more massive systems hosting intermediate and older populations. The two cosmologically young dwarfs are lit up by tidal interactions, have compact morphologies, and have metallicities and cold gas fractions similar to the relatively quiescent, extremely metal-deficient dwarf population that includes the recently-discovered Leo P. Metal-enriched galactic outflows produce sub-solar effective yields and pollute with heavy elements a Mpc-size region of the intergalactic medium, but are not sufficient to completely quench star formation activity and are absent in the faintest dwarfs. Within the limited size of the sample, our simulations appear to simultaneously reproduce the observed stellar mass and cold gas content, resolved star formation histories, and metallicities of field dwarfs in the Local Volume.

Motivated by the observed connection between molecular hydrogen (H2) and star formation, we present a method for tracking the non‐equilibrium abundance and cooling processes of H2 and H2‐based star formation in smoothed particle hydrodynamic simulations. The local abundances of H2 are calculated by integrating over the hydrogen chemical network. This calculation includes the gas phase and dust grain formation of H2, shielding of H2 and photodissociation of H2 by Lyman–Werner radiation from nearby stellar populations. Because this model does not assume equilibrium abundances, it is particularly well suited for simulations that model low‐metallicity environments, such as dwarf galaxies and the early Universe. We further introduce an explicit link between star formation and local H2 abundance. This link limits star formation to ‘star‐forming regions’, represented by areas with abundant H2. We use simulations of isolated disc galaxies to verify that the transition from atomic to molecular hydrogen occurs at realistic densities and surface densities. Using these same isolated galaxies, we establish that gas particles of 104 M⊙ or less are necessary to follow the molecular gas in this implementation. With this implementation, we determine the effect of H2 on star formation in a cosmological simulation of a dwarf galaxy. This simulation is the first cosmological simulation with non‐equilibrium H2 abundances to be integrated to a redshift of zero or to include efficient supernova feedback. We analyse the amount and distribution of star formation in the galaxy using simulated observations of the H i gas and in various optical bands. From these simulated observations, we find that our simulations are consistent with the observed Tully–Fisher, global Kennicutt–Schmidt and resolved Kennicutt–Schmidt relations. We find that the inclusion of shielding of both the atomic and molecular hydrogen and, to a lesser extent, the additional cooling from H2 at temperatures between 200 and 5000 K increases the amount of cold gas in the galaxies. The changes to the interstellar medium (ISM) result in an increased amount of cold, dense gas in the disc of the galaxy and the formation of a clumpier ISM. The explicit link between star formation and H2 and the clumpier ISM results in a bluer galaxy with a greater spatial distribution of star formation at a redshift of zero.

We examine the present-day total stellar-to-halo mass (SHM) ratio as a function of halo mass for a new sample of simulated field galaxies using fully cosmological, ΛCDM, high resolution SPH + N-Body simulations. These simulations include an explicit treatment of metal line cooling, dust and self-shielding, H 2 based star formation and supernova driven gas outflows. The 18 simulated halos have masses ranging from a few times 10 8 to nearly 10 12 M ⊙ . At z=0 our simulated galaxies have a baryon content and morphology typical of field galaxies. Over a stellar mass range of 2.2 × 10 3 -4.5 × 10 10 M ⊙ we find extremely good agreement between the SHM ratio in simulations and the presentday predictions from the statistical Abundance Matching Technique presented in Moster et al. (2012). This improvement over past simulations is due to a number systematic factors, each decreasing the SHM ratios: 1) gas outflows that reduce the overall SF efficiency but allow for the formation of a cold gas component 2) estimating the stellar masses of simulated galaxies using artificial observations and photometric techniques similar to those used in observations and 3) accounting for a systematic, up to 30% overestimate in total halo masses in DM-only simulations, due to the neglect of baryon loss over cosmic times. Our analysis suggests that stellar mass estimates based on photometric magnitudes can underestimate the contribution of old stellar populations to the total stellar mass, leading to stellar mass errors of up to 50% for individual galaxies. These results highlight the importance of using proper techniques to compare simulations with observations and reduce the perceived tension between the star formation efficiency in galaxy formation models and in real galaxies.